Kotlin Interview Questions

Kotlin is a modern, statically typed programming language that runs on the Java Virtual Machine (JVM) and can be used for Android development, server-side development, and more. It is known for its concise syntax, safety features, and interoperability with Java. 

If you're preparing for a job interview for Kotlin, it's essential to understand its core concepts, features, and best practices. This blog post covers some of the most commonly asked Kotlin interview questions and answers to help you prepare effectively for your job interviews.

1. What is Kotlin?

Answer: Kotlin is a statically typed, general-purpose programming language developed by JetBrains. It runs on the Java Virtual Machine (JVM) and can also be compiled into JavaScript and native code. 

Kotlin is designed to be fully compatible with Java, making it a popular choice for Android development and other Java-based projects.

2. What are the main features of Kotlin?

Answer: Key features of Kotlin include:

  • Concise Syntax: Kotlin reduces boilerplate code, making programs more readable and maintainable.
  • Null Safety: Kotlin provides built-in null safety features to prevent null pointer exceptions.
  • Interoperability: Kotlin is fully interoperable with Java, allowing the use of existing Java libraries and frameworks.
  • Extension Functions: Kotlin allows you to extend existing classes with new functionality without modifying their source code.
  • Coroutines: Kotlin provides support for coroutines, which simplifies asynchronous programming.

3. What is the difference between val and var in Kotlin?

Answer: In Kotlin, val and var are used to declare variables:

  • val: Declares a read-only variable (similar to final in Java). Once a value is assigned, it cannot be changed.
    val name = "John"
    // name = "Doe" // This will cause a compilation error
  • var: Declares a mutable variable. Its value can be changed after the assignment.
    var age = 25
    age = 30 // This is allowed

4. How does Kotlin handle null safety?

Answer: Kotlin provides built-in null safety features to prevent null pointer exceptions. By default, variables cannot be null. To allow null values, you must explicitly mark the variable type with a question mark (?).


var name: String = "John" // Non-nullable
// name = null // Compilation error

var nullableName: String? = "John" // Nullable
nullableName = null // Allowed

To safely access nullable variables, you can use safe calls (?.), the Elvis operator (?:), or the !! operator for asserting non-null.


val length = nullableName?.length // Safe call, returns null if nullableName is null
val lengthOrDefault = nullableName?.length ?: 0 // Elvis operator, returns 0 if nullableName is null
val nonNullLength = nullableName!!.length // Non-null assertion, throws exception if nullableName is null

5. What are extension functions in Kotlin?

Answer: Extension functions in Kotlin allow you to add new functionality to existing classes without modifying their source code. They are defined using the fun keyword, followed by the class name, a dot, and the function name.


fun String.isEmail(): Boolean {
    return this.contains("@") && this.contains(".")

val email = "[email protected]"
println(email.isEmail()) // Output: true

6. What are data classes in Kotlin?

Answer: Data classes in Kotlin are used to hold data. They automatically generate useful methods, such as equals(), hashCode(), toString(), and copy(). Data classes are declared using the data keyword.


data class User(val name: String, val age: Int)

val user = User("John", 30)
println(user) // Output: User(name=John, age=30)

7. What is a companion object in Kotlin?

Answer: A companion object in Kotlin is a singleton object that is associated with a class. It can be used to hold static members and factory methods. Companion objects are declared using the companion object keyword.


class MyClass {
    companion object {
        fun create(): MyClass = MyClass()

val instance = MyClass.create()

8. How do you create a singleton in Kotlin?

Answer: In Kotlin, you can create a singleton using the object keyword. An object declaration defines a class and creates an instance of it.


object MySingleton {
    fun showMessage() {
        println("Hello, Singleton!")

MySingleton.showMessage() // Output: Hello, Singleton!

9. What are coroutines in Kotlin?

Answer: Coroutines in Kotlin are a feature that simplifies asynchronous programming. They allow you to write asynchronous code in a sequential style, avoiding callback hell. Coroutines are lightweight and can be suspended and resumed without blocking threads.


import kotlinx.coroutines.*

fun main() = runBlocking {
    launch {

10. What is a sealed class in Kotlin?

Answer: A sealed class in Kotlin is a special kind of class that restricts the hierarchy to a specific set of subclasses. Sealed classes are useful for representing restricted class hierarchies, such as when modeling finite state machines.


sealed class Result
data class Success(val data: String) : Result()
data class Error(val error: String) : Result()

fun handleResult(result: Result) {
    when (result) {
        is Success -> println("Success: ${result.data}")
        is Error -> println("Error: ${result.error}")

11. What is the difference between == and === in Kotlin?

Answer: In Kotlin, == is used for structural equality (equivalent to equals() in Java), while === is used for referential equality (checks if two references point to the same object).


val a = "hello"
val b = "hello"
val c = a

println(a == b) // Output: true (structural equality)
println(a === b) // Output: false (referential equality)
println(a === c) // Output: true (referential equality)

12. How do you handle exceptions in Kotlin?

Answer: Exception handling in Kotlin is similar to Java. You use try, catch, and finally blocks to handle exceptions.


try {
    val result = 10 / 0
} catch (e: ArithmeticException) {
    println("Exception caught: ${e.message}")
} finally {
    println("This block is always executed")

13. What are higher-order functions in Kotlin?

Answer: Higher-order functions are functions that take other functions as parameters or return functions as results. They are a key feature of functional programming.


fun higherOrderFunction(operation: (Int, Int) -> Int): Int {
    return operation(2, 3)

fun main() {
    val sum = higherOrderFunction { a, b -> a + b }
    println(sum) // Output: 5

14. What is the inline function in Kotlin?

Answer: Inline functions are functions that the compiler expands at the call site to reduce the overhead of function calls, especially for higher-order functions.


inline fun inlineFunction(block: () -> Unit) {
    println("Before inline function")
    println("After inline function")

fun main() {
    inlineFunction {
        println("Inside inline function")

15. What is a lambda expression in Kotlin?

Answer: A lambda expression in Kotlin is an anonymous function that can be treated as a value and passed around. It is defined using curly braces {} and can capture variables from its surrounding scope.


val sum: (Int, Int) -> Int = { a, b -> a + b }
println(sum(2, 3)) // Output: 5

16. How do you define a default parameter value in Kotlin?

Answer: In Kotlin, you can define default values for function parameters. If a value is not provided for a parameter, the default value is used.


fun greet(name: String = "Guest") {
    println("Hello, $name!")

greet() // Output: Hello, Guest!
greet("John") // Output: Hello, John!

17. What is the use of lateinit keyword in Kotlin?

Answer: The lateinit keyword is used to declare a non-null variable that will be initialized later. It can only be used with var and is often used for dependency injection or for initializing properties in Android activities.


lateinit var name: String

fun initializeName() {
    name = "John"

fun printName() {
    if (::name.is

Initialized) {
    } else {
        println("Name is not initialized")

18. How do you make a class immutable in Kotlin?

Answer: To make a class immutable in Kotlin, declare all properties as val (read-only) and avoid providing setters.


data class User(val name: String, val age: Int)

val user = User("John", 30)
// user.name = "Doe" // This will cause a compilation error

19. How do you create a singleton in Kotlin without using the object keyword?

Answer: You can create a singleton in Kotlin using a companion object within a class and ensuring the constructor is private.


class Singleton private constructor() {
    companion object {
        val instance: Singleton by lazy { Singleton() }

val singletonInstance = Singleton.instance

20. What is destructuring in Kotlin?

Answer: Destructuring in Kotlin allows you to unpack properties from an object into separate variables. It is commonly used with data classes.


data class User(val name: String, val age: Int)

val user = User("John", 30)
val (name, age) = user

println("Name: $name, Age: $age") // Output: Name: John, Age: 30


Understanding Kotlin's core concepts, features, and best practices is crucial for any developer working with Kotlin. This blog post covered some of the most commonly asked Kotlin interview questions, helping you prepare effectively for your next interview. By mastering these concepts, you will be well-equipped to tackle any Kotlin-related challenges you may encounter.